Prime Number Cicadas

The cacophony by the lake in western New Jersey was deafening. Which was louder, the voices of my 30-odd family members at our biennial reunion, or the din of millions of cicadas? Both groups were noisy, but the cicadas, making up for 17 years of celibate residency in holes in the ground, were the loudest. Well, they had to be. The male of the species Cicadidae magicicada has but four weeks or so to attract females, mate, mate and mate ... and die.

Once inseminated, a female cicada climbs up the trunk of a nearby tree, cuts V-slits in young twigs, lays about 20 eggs and repeats the process 30 or so times for a total of about 600 eggs before she, too, dies. The eggs hatch about eight weeks later, and the newborn nymphs fall to the ground where they burrow down about a foot to start their 17-year troglodyte existence. Most never make it; 98 percent die in the first two years. The survivors live it up on a diet of xylem fluids found in the roots of deciduous woodland trees.

Finally, on a single spring evening when the soil temperature is just right, the nymphs emerge in their multitudes — up to two million per acre! In southern states, E-day is late April to early May, while broods in northern states emerge from late May to early June. A few days later, after a series of moltings, the adult males congregate in vast choruses of lust. Unlike crickets, which produce their signature chirping by rubbing their wings together, cicadas vibrate their entire corrugated exoskeletons, or timbals. The noise of millions of these creatures can be thunderous; they produce some of the loudest decibel ratings (up to 107 dB) of any insects.

The question, of course, is why 17 years? Or 13 years, in related species of periodic cicadas? The most common answer given is predator saturation. When all the nymphs in one area emerge simultaneously, birds, squirrels and other predators soon gorge themselves, leaving the survivors to breed in peace. The prime number pattern (13 and 17 are both prime) optimizes this saturation strategy by keeping the cicadas out of step with predators. If instead cicadas emerged every, say, 12 years, a predator might well evolve a strategic periodicity of its own, synchronizing its breeding cycle to two, three, four or six years (factors of 12); the 13- and 17-year cycles avoid a matching predator periodicity. Predators on the same cycle would decimate the cicadas. Also, the two cycles of 13 and 17 years helps keep "rival" cicada species separate, again optimizing survival rates by minimizing hybridization, which could throw off their timing, again putting them at greater risk from predators. The 13-year cicadas are found mostly in southern states, and the 17-year variety stays in northern states, with very little overlap.

None of which really explains the magic of these 13- and 17-year cycles: How on earth did cicadas evolve to know when it's the correct year to emerge? I didn't see any tiny insect calendars, yet somehow all the cicadas we recently heard and saw knew it was 2013, and they'll know when 2030 rolls around. Turns out it's all controlled by a single gene locus that counts out those 13 or 17 years.

I wouldn't normally enjoy being surrounded by millions of lusty, noisy 17-year olds, but in this case I had to make an exception. For me, the cacophony is a paean to the genius of evolution.